def prost(x):
 k=0
 for d in range(1,x+1):
    if x%d==0:
       k+=1
 if k==2:
        return True
 else:
        return False
maxx=0
num=int(input())
if num>=1000 and num<=9999:
   n1=str(num)
   for cifra in range(len(n1)):
    if prost(int(n1[cifra])):
        maxx=max(maxx,int(n1[cifra]))
   for chislo in range(len(n1)-1):
    if prost(int(n1[chislo]+ n1[chislo+1])) == True:
        maxx=max(maxx,int(n1[chislo]+ n1[chislo+1]))
   for chislo in range(len(n1)-2):
    if prost(int(n1[chislo]+ n1[chislo+1]+n1[chislo+2])) == True:
        maxx=max(maxx,int(n1[chislo]+ n1[chislo+1]+n1[chislo+2]))
   for chislo in range(len(n1)-3):
    if prost(int(n1[chislo]+ n1[chislo+1]+n1[chislo+2]+n1[chislo+3])) == True:
        maxx=max(maxx,int(n1[chislo]+ n1[chislo+1]+n1[chislo+2]+n1[chislo+3]))
    print(maxx)
else:
    print(0)
